hahah @Elliandre them ordering parts by themselves would be handy (no pun intended). That way I didn't have to pay attention to what parts they need for any upgrade.
And yes, still a mystery it seems. My scientist came home from work in a confident mood. I tell him to go use the toilet and then upgrade it with a bidet. Almost as soon as he starts the upgrade, he adds "Pee like a champion" to his queue. He just went, his bar is absolutely fully green, its not necessary to go again :| I remove the action, it almost instantly pops back in queue.
I've tried to search AHQ for it, but its difficult for me to find anything, because I don't really know what words to use in the search. Its such a vague "issue", I can't find a spesific term for it.